This invention relates to a frequency stable pulsed laser.
Laser designs account for frequency stabilization to achieve accuracy and high signal-to-noise ratio (SNR). Some laser designs use a master-slave configuration to achieve frequency stabilization. One implementation of a laser design is a Doppler Light Detection and Ranging (LIDAR) system that measures the velocity of a target. The targets are particles in the air, which are used to measure wind velocity. A transmitter transmits light that hits a moving target. The light reflects or scatters back off the target. The wavelength or frequency of the reflected light changes due to the Doppler shift caused by the moving target. The Doppler LIDAR system then determines the velocity of the target from the change in the wavelength.
The frequency stabilization of the Doppler LIDAR system is critical to achieve velocity measurements with high accuracy and high SNR. Particularly, frequency stabilization having low chirp is beneficial to achieve a high SNR. In coherent detection using a master slave configuration, a slave laser uses a local oscillator or master laser for a reference frequency. The transmitted pulse frequency is shifted from the local oscillator frequency to detect zero velocity and retrieve the sign of the reflected signal. In prior systems, single-frequency operation of the pulsed laser has used injection seeding. Injection seeding is a technique in which a pulsed laser is locked to the frequency of a continuous wave laser by flooding the pulsed laser cavity with continuous wave laser photons of the desired frequency prior to pulse generation.
In order to achieve master-slave configuration frequency stabilization, one prior system locked the master and slave resonator frequency to an external interferometer. Another system minimized the Q-switch build-up time for frequency stabilization. The offset between master and slave frequency was also observed to stabilize frequency. A ramp-and-fire technique, dither lock-in technique, and the observation of the resonance passively induced by flashlamp pump pulse have also been used for frequency stabilization. In a rough environment such as on a ship, stable operation of all these prior systems is difficult to achieve. Also, frequency chirp exists in these prior systems.
FIG. 1 depicts a laser phase and frequency stabilization system using an optical resonator in the prior art. The laser phase and frequency stabilization system uses phase modulation for frequency stabilization. This system stabilizes a continuous wave laser using a highly stable external reference cavity.
The invention solves the above problems by stabilizing the frequency of a pulsed slave laser using the master laser frequency to stabilize a cavity in the slave laser. The slave laser includes an optical modulator, a cavity, a cavity modifier, and an output generator. The cavity includes an end reflector, a laser generator, an optical injector, and an output coupler. The optical modulator receives a continuous wave laser signal that includes a carrier frequency. The optical modulator then modulates the continuous wave laser signal to generate two sidebands around the carrier frequency. The laser generator generates a first laser signal in the cavity. The optical injector then injects the continuous wave laser signal with the first laser signal. The output generator generates an output signal based on the continuous wave laser signal. The cavity modifier then modifies a length of the cavity based on the output signal wherein the cavity is in resonance with the frequency of the continuous wave laser signal. The output coupler then transmits the pulsed first laser signal from the output coupler.
In one embodiment, a Faraday isolator isolates the continuous wave laser signal before injection. In another embodiment, the laser generator pumps longitudinally two pump lights into a crystal to generate the first laser signal. In yet another embodiment, the optical injector injects the continuous wave laser signal with the first laser signal from a side at a perpendicular polarization of a path that the first laser signal propagates on.
The slave laser advantageously transmits a laser signal with stable frequency. In one embodiment, the frequency stability is approximately 0.2 MHz rms. Also, the slave laser transmits the laser signal with non-detectable chirp. The non-detectable chirp allows accurate velocity measurements with high SNR ratios. One example for velocity measurements is the measurement of wind.